Fretting fixture for high-cycle fatigue test machines

Abstract

A fretting fixture accessory for a_test machine ( 10 ) that induces high-cycle fatigue (at kilohertz vibration rates) in a specimen of a material under test. The fretting fixture ( 20 ) is clamped to the test specimen ( 21 ), for the purpose of testing for fretting damage. The fixture ( 20 ) is designed to provide both the normal and shearing forces that result in fretting damage.

What is claimed is:  
     
       1. A fretting fixture for attachment to a specimen mounted in a fatigue testing machine, comprising: 
       a clamping frame having at least two loading rods and two clamping beams, such that each loading rod extends through a clamping beam at each end;  
       a pair of inertial masses inside said clamping frame, one adjacent to each clamping beam such that an air gap exists between opposing surfaces of the inertial masses within the frame; and  
       a fretting contact piece attached to at least one of the opposing surfaces;  
       the clamping frame operable to clamp the inertial masses to the specimen, one on each side of the specimen, with the fretting contact piece between one of the inertial masses and the specimen, such that the fretting fixture is supported by the specimen and moveable in space with the specimen.  
     
     
       2. The fretting fixture of  claim 1 , further comprising at least one accelerometer attached to the fretting fixture. 
     
     
       3. The fretting fixture of  claim 1 , further comprising at least one strain gauge attached to the fretting fixture. 
     
     
       4. The fretting fixture of  claim 1 , wherein the inertial masses are made at least primarily of a high density material. 
     
     
       5. A method of testing, for fretting stress, a specimen already mounted in a test machine, comprising the steps of: 
       placing a fretting piece against the test specimen, the fretting piece having a contact area on a surface of the test specimen;  
       clamping an inertial mass on each side of the combination of the test specimen and the fretting piece, thereby resulting in a fretting fixture assembly of the inertial masses clamped to the specimen, one on each side of the specimen, with the fretting contact piece between one of the inertial masses and the specimen, such that the fretting fixture is supported by the specimen and moveable in space with the specimen; and  
       activating the test machine thereby causing the test specimen and fretting fixture to oscillate.  
     
     
       6. The method of  claim 5 , further comprising the step of using the test machine to apply a static load on the test specimen. 
     
     
       7. The method of  claim 6 , wherein the test specimen is placed inside a test machine frame, and step of applying a static load is performed with force applied to the frame. 
     
     
       8. The method of  claim 6 , wherein said step of applying a static load places said test specimen in tension. 
     
     
       9. The method of  claim 5 , further comprising the step of using the test machine to apply a dynamic load on the test specimen. 
     
     
       10. The method of  claim 9 , wherein the dynamic load is applied with one or more dynamic actuators. 
     
     
       11. The method of  claim 5 , wherein said test specimen oscillations are translation motions along its length axis. 
     
     
       12. The method of  claim 5 , further comprising the step of placing a nonfretting contact pad on the opposite side of said test specimen in contact with the fretting piece between one of the inertial masses and the specimen. 
     
     
       13. The method of  claim 5 , wherein the clamping step is performed by placing a clamping frame around the assembly comprised of the inertial masses, the test specimen, and the fretting piece. 
     
     
       14. The method of  claim 5 , wherein the said test specimen has a lengthwise vibrational axis and wherein the said fretting fixture is oriented normal to the vibrational axis with said fretting piece positioned and clamped at the midpoint of the test specimen. 
     
     
       15. The method of  claim 5 , wherein said clamping frame used in said clamping step has sensing means for determining the clamping force and the shear force at the contact between said fretting piece and said test specimen.